You've been using soaps and shampoos for (probably) as long as you can remember. But do you know how they're made?
In this article we explore some of the history of soap making and take a deep dive into the science behind how mixing just the right set of ingredients results in something that will help keep you clean and smelling great.
A brief history of soap making
It is thought, like many early human inventions, the first soaps and shampoos were made by accident.
There's no consensus about who invented them, nor where it happened.
One Roman legend has it that soap was named after Mount Sapo, where animals were sacrificed. Supposedly rainwater washed ashes and melted animal fats (tallow) from the sacrifices into the river below the mountain, where people washed themselves and their clothes. They found the river foamed and cleaned well as the tallow and ash reacted with the water to become a form of soap.
The Sumerians used a slurry of ashes and water to remove grease from raw wool and cloth before it could be dyed.
The alkali nature of the ashes would have reacted with the fats in the grease on the wool to turn them into a slippery, soap like, substance, which would have in turn made much of the remaining fats dissolve, making the process of cleaning the wool and cloth faster and easier.
The first recorded evidence of soap production (or at least something like it) dates back to around 2800 BC in ancient Babylon.
A formula for soap consisting of water, alkali, and cassia oil was written on a Babylonian clay tablet around 2200 BC.
What connects all the differing accounts of soap’s invention is a process of combining fats, alkali and water to produce something that helps keep things, including us humans, clean and smelling nice.
Fats + Alkali + Water = soap
Soap making is a relatively simple example of a chemical reaction.
Sodium hydroxide is an alkali. It's a highly reactive substance. This is because at a molecular level it is highly charged causing it to attract other molecules to it with enough force to break bonds between atoms, allowing it to move atoms (or groups of atoms) from one molecule to another.
Triglycerides are found in oils and are a type of fat. Triglyceride molecules are composed of three fatty acids joined to a glycerol. They are very common in animal and plant oils because both animals and plants use them as a way of storing fats.
Your own body stores triglycerides in fat cells when you consume more calories than you need to use straight away...
Different oils from different animals and plants have slightly differently configured triglycerides. Some are formed of longer chains of atoms. Some are shorter. Ghee (clarified butter), for example contains Butyric acid, which is a short fatty acid.
Oleic acid, a fatty acid found in olive, macadamia nut, corn and groundnut oils has a long configuration.
To make soap the sodium hydroxide is dissolved in water to make lye which is then combined with triglycerides to encourage a chemical reaction known as saponification (latin for the creation of soap).
Essentially the reaction causes the highly reactive sodium hydroxide molecules to break up triglycerides molecules. They first combine, split and then recombine in slightly different configurations to form new soap molecules.
For every triglyceride molecule in the mixture, three soap molecules and one glycerol molecule are produced.
The sodium hydroxide molecules are all consumed by the reaction, making it safe to touch and use soap.
The spare glycerol molecules left from the reaction remain in the soap as useful conditioning and lather boosting ingredients.
As different oils contain different fatty acids (some longer, shorter, etc), using different oils in soap will result in the soaps having slightly different chemical compositions.
Those different compositions are responsible for whether a soap is particularly hard, or soft. Or whether it lathers well or not. Or whether it conditions your skin or hair when used.
While the chemical reaction that forms soap is relatively simple there are some essential elements of the process that need to be controlled in order to make the resulting soap safe to use.
Firstly: consume all the NaOH
The reaction must consume all the sodium hydroxide that was added to the mixture. If this is not fully consumed the soap will be dangerous to use. It will irritate and potentially burn you.
To ensure it is fully consumed the mixture must contain more oil than is needed to react with the sodium hydroxide fully. Each oil used in soap making has a known saponification value (or SAP value). When formulating a recipe for a soap, you need to know the SAP value of each oil you'll be adding, from which you can calculate how much sodium hydroxide you need to make soap. A recipe should use between 80%-90% of the sodium hydroxide that would be needed to cause saponification for all the oils in the mixture. The result is an excess of oil that is not consumed by the sodium hydroxide, is not turned into soap, and is known as the soap's "superfats".
Secondly: mix it right
The second element of the reaction that needs to be carefully controlled is how the fatty acids and sodium hydroxide are mixed.
A soap maker must ensure that all the sodium hydroxide molecules have come into contact with the oils. To do this, all the ingredients in the soap must be liquids so they can be mixed thoroughly. The sodium hydroxide is dissolved in water to make it a liquid and any oils that are solids at normal room temperatures have to be heated and melted to make sure they too are fully liquid before being mixed.
Finally: control the temperature
The final element that needs to be controlled is the temperature. When you heat chemicals, they react more quickly. In soap making as the sodium hydroxide solution and liquid oils are combined they start to thicken and turn solid. If that happens before the solutions can be fully mixed, the soap will harden too soon resulting in pockets of sodium hydroxide that are not in contact with any fatty acids, and therefore don't react. That's bad (see point 1 above).
As the ingredients are mixed you are looking to achieve a stable emulsion where all of the sodium hydroxide solution has met, and reacted with, a fatty acid (oil). If you don't mix enough this won't happen and eventually the mixture will separate into distinct layers of soap, oil and water.
However, if you mix the ingredients for too long they will harden and the soap becomes impossible to mould into shape.
At this mixing stage of the soap making process there are two options for how to proceed:
- In what's know as "hot process" you now heat the mixture to keep it liquid for longer while the saponification reaction continues.
- In what's know as the "cold process" the mixture is poured into moulds to cool and continue its saponification process over a longer period of time.
We use the cold process method to make our shampoo bars as our ingredients are more sensitive to heat than in traditional soap bars.
With the cold process the soap (or in our case at Tidy shampoo) bars are left to cure and set for anywhere between 4-6 weeks (we cure our shampoo for a minimum of 6 weeks). During this time the saponification process continues but gradually slows as all the sodium hydroxide is consumed and turned to soap.
Once the bars have fully cured they can be wrapped and used safely.
As this point you might be thinking to yourself; "Great! So now I know how soap is made, but how do you make shampoo?!"
It's the same process. The difference between a soap and a shampoo bar is basically how much superfat there is in the mixture, and which oils are used in the recipe. The chosen oils and quantities of superfats determine how cleansing / conditioning the bars are for your hair.
Our Tidy solid shampoo bars for men are formulated to be suitable for most hair types, giving a deep clean while mildly conditioning your hair. You can control how deeply it cleans your hair by using more or less each time you wash.